If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.

Constant Current vs. PWM dimming Revealed

This chart is the CIE color chart with the Constant Current dimming and PWM dimming, with the Luxeon bins shown:

Here is an exploded view of just the data, note the dimming % is listed, % for PWM is the PWM duty cycle, % for Constant Current is the % of the max current:

In the above chart, take note of the percent where the constant current and PWM dimming split, about 50% for the Constant Current. A person could dim with constant current to 50%, then utilize PWM, to minimize color shift, yet pick up the efficiency increase.

Here is the efficiency difference:

Yet that chart only shows half the story, here is the efficiency gain at each step, I was rather surprised here:

Basicially it all boils down to Current Dimming is alot more efficient than PWM dimming (so you get considerably more run time on down to 0.5% current dimming, but PWM dimming results in less color shift. A combination of the two approaches could sure be interesting indeed.

FYI, the current level this Luxeon was ran at is 380.1 mA = 100% Also, the Luxeon emitter slug had a hole drilled into the side and a type-K thermalcouple inserted, the slug temperature variation over the whole test was a maximum of 2.3 degrees Celcius (yes, a large aircooled heatsink was utilized for this). The same emitter is utilized for all sets of data.

Re: Constant Current vs. PWM dimming Revealed

PWM requires power for switching. I'm surprised the efficiency delta isn't just a constant all the way up to 100% power, where they're both just constant-current situations.

Switch power on, switch it off. The hertz should be the same no matter what the pulse width, so the penalty seems like it should be linear.

...but I'm thinking electrically, not optically. I'd think that the optical difference would be negligible with a pulse width that's long enough to reduce LED response time penalty to a negligible function of the pulse.

Re: Constant Current vs. PWM dimming Revealed

Right, but the Luxeon emitter becomes significantly more efficient at lower current levels.

Where with straight PWM, at the same current, you are hitting the emitter with full current.

The info is right on the Luxeon datasheet, if you look carefully.

Next to measure the difference utilizing 1 Amp.

I am expecting even a larger efficiency difference, we will see.

The efficiency gain of current dimming of up to 136% was rather surprising, I was not expecting that large of a gain, more like 15%, but the datasheet doesn't go down to 11.4 mA...http://www.lumileds.com/pdfs/DS45.PDF

It's kinda cool, how the PWM dimming turned out so flat, just like it should.

BTW, each individual measurement point was taken 15 times and the 15 samples were averaged, to increase accuracy. (in otherwords, 270 measurements were taken, thank goodness for computers).

For the technogeeks that wanna be in the know, the frequency that the PWM circuit was ran at was 133 Hz, which means the pulse width at the lowest level (0.05%) was 0.000003759 seconds (or 3.759e-6 seconds).

Re: Constant Current vs. PWM dimming Revealed

Okay, I ran some numbers at 1140mA, such as you'd have with a Li-Ion, and using a PWM to dim the LED.

First off, at the higher current level, the tint variation increases substantially:

A close up at 1140mA:

And we have the increased light output efficiency of current dimming vs. PWM dimming:

It really adds up alot at the lower dimming levels:

On a side note, I noticed that the curve fit for what is shown on the LumiLEDs datasheet (output vs. current), when measuring a blue emitter, ends up with nearly an exact match , but the curve is more for a white emitter.

Re: Constant Current vs. PWM dimming Revealed

Hi Newbie - Great info as usual. Also as usual, I am trying to really grasp some of the more subtle aspects of your data and analysis.

I always try to compare the data and value of an advancement to the most basic way to manage the current of an LED driver (a set of resistors at 2, 10, and 25 ohms in line with 2 x 123 cells.)

Would it be correct to interpret from the information that at dimming near the 10 % x 1100ma point, a resistor setup would be more efficient than PWM ? Of course, color control is something else entirely.

Sorry if this question is entirely missing the point of your post. [img]/ubbthreads/images/graemlins/confused.gif[/img]

Re: Constant Current vs. PWM dimming Revealed

HarryN said:
Hi Newbie - Great info as usual. Also as usual, I am trying to really grasp some of the more subtle aspects of your data and analysis.

I always try to compare the data and value of an advancement to the most basic way to manage the current of an LED driver (a set of resistors at 2, 10, and 25 ohms in line with 2 x 123 cells.)

Would it be correct to interpret from the information that at dimming near the 10 % x 1100ma point, a resistor setup would be more efficient than PWM ? Of course, color control is something else entirely.

Well, if you mean 1140mA * 10% = 114mA for current dimming,
vs. a 10% PWM duty cycle with 1140mA pulses, yes, most definitely. If 47.5% of the power to the LED went up in the resistor, you'd be equal for efficiency, with more color tint drift. You'd probably pick up a tad of additional efficiency, since the I^2*R losses in the battery would be lower too.

If you have more than 47% of the power going up in the resistor, the PWM would be more efficient.

If you utilized a switcher that was over 53% efficient, you'd have less losses than a PWM, and most switchers are considerably more efficient than that, even the simple Zetex ones do better than that.

Even at 85% On duty-cycle and below, the NextGen converter that dat2zip did, and setting it for a lower output current level, would do better than a PWM curcuit, due to the efficieny losses emitter when you whack it with 1140mA using a PWM circuit.

So you are going to have to figure out your resistor value, and compute the power you have to burn up as heat, to know for sure.

Re: Constant Current vs. PWM dimming Revealed

Nice work, Doug! I note that the emitter x-y chromacity coordinates fall outside the bin structure of the current Luxeon binning scheme. Do you know the bin of the emitter tested? BTW, On the Nichia datasheets for their high power white LEDs they provide some graphs related to those that you have done. They provide graphs of x-y chromacity as functions of temp and also of drive current.

Re: Constant Current vs. PWM dimming Revealed

Are there any lumen maintenance considerations between a PWM dimmed LED (from a high level of current) as compared to a constant current set up that is driving at a lower current level, comparable in output to the dimmed PWM circuit? If heat is the primary antagonist to the lumen manitneance then I am assuming that the only difference would be due to the delta in efficiencies between the two systems. However is there additional degradation to the life of the LED as a function of the current level beyond the thermal issues?

EDIT: I recall PK stating that there were a number of reasons and merits for going with the various constant current levels on the U2 instead of going with PWM but he did not elaborate. Even though there are the short comings in color shift, SF chose to go with the variationin current level over a duty cycle. [img]/ubbthreads/images/graemlins/icon3.gif[/img] [img]/ubbthreads/images/graemlins/thinking.gif[/img]

Re: Constant Current vs. PWM dimming Revealed

[ QUOTE ]McGizmo said:
However is there additional degradation to the life of the LED as a function of the current level beyond the thermal issues?

[/ QUOTE ]

Don, the Luxeon Application Brief AB25, Luxeon Reliability, seems to suggest so. It would be hard from this AB alone to say for sure whether CC or PWM would be preferred from a lumen maintenance standpoint but if the "all else equal" condition stipulates equal luminous output I would intuitively suspect that the CC method would be preferred.

Re: Constant Current vs. PWM dimming Revealed

Well. this definitely bodes well for my LDO driver on my upcoming variable nano (which dims down to zero).. i think that it only takes like 80 micro amps to drive my circuit, so when dialed down to low double-digits i should be able to get double-digit hours of runtime with the varinano.

Re: Constant Current vs. PWM dimming Revealed

Doug S said:
Nice work, Doug! I note that the emitter x-y chromacity coordinates fall outside the bin structure of the current Luxeon binning scheme. Do you know the bin of the emitter tested? BTW, On the Nichia datasheets for their high power white LEDs they provide some graphs related to those that you have done. They provide graphs of x-y chromacity as functions of temp and also of drive current.

I'm aware of the Nichia datasheet, haven't had a chance to look at it and compare. I don't know the bin, this BW01 is before hounding Future to give us the binning data on the emitters. It is a rather yellow emitter, fyi.

When you current dim, the wavelength of a blue LED shifts alot, see below:

Re: Constant Current vs. PWM dimming Revealed

This is one of the best threads I read for a while. As I cannot contribute in an adequate way here I simply say "Thanx" to those who actually did the work !
[img]/ubbthreads/images/graemlins/thumbsup.gif[/img] [img]/ubbthreads/images/graemlins/bowdown.gif[/img]
bernhard

Re: Constant Current vs. PWM dimming Revealed

This is what the blue LED underneath the phosphor is actually doing, as far as wavelength shift, note how it stays put during PWM dimming:

Here is what the spectral output is actually doing, notice how it gets less pure when current dimming:

To help visualize things, here is a visual spectral plot of the blue LED. Notice how the spectral plot for each is not symetrical on each side of the wavelength peak, and more is on the green side. Next, note how the peak has moved, with the different current levels. Finally, check out the width of each plot, and notice how it broadens out at low current levels.

In contrast, with PWM dimming, the dominant and the peak wavelength do not change, and when you dim it the spectrum actually narrows, reverse of what current dimming does. And it doesn't shift in wavelength left or right. See below:

Re: Constant Current vs. PWM dimming Revealed

In the name of efficacy, I say bring on the CC and let the color shift be hanged! [img]/ubbthreads/images/graemlins/grinser2.gif[/img] Surely it is poetic that the shift is towards the warmer levels for the more intimate settings. [img]/ubbthreads/images/graemlins/nana.gif[/img]

Re: Constant Current vs. PWM dimming Revealed

This is the difference in the relative output when you PWM dim, shown in a spectral power plot:

Here is the difference of Current dimming in a spectral power plot:

Here below is the difference of the 0.05% power input levels for current dimming vs. PWM dimming. Take careful note of how with the current dimmed plot, how the blue peak shifted to a longer wavelength towards the red, and how the output of the phosphor (the big hump running from cyan through green to red), increased it's relative output (went higher) in relation to the blue peak at 100%.

Not every emitter has the same wavelength blue LED die in it (think of the luxeon lottery but in blue), and depending on where it's wavelength starts at, in relationship to the peak phophor conversion efficiency wavelength for the blue, emitters in the same batch and binning will do different things.

I tried to hold the slug temperature as constant as possible. But in a flashlight, it usually isn't possible to do provide such extreme cooling. Additional changes begin to occur once you start changing the temperature.

I hope this has been educational and enlightening for the general CPF crowd.

Re: Constant Current vs. PWM dimming Revealed

Interesting data on the blue lux wavelength shift with current. If we take the 380mA as 100% and the 7.6mA as 2%, the shift from 2% to 100% is 5nm. Putting this into perspective, this is the width of a single color bin for the blue emitters. AB21 defines 6 bins for blue emitters, each 5nm wide. There is also a tempco associated with wavelength. The luxeon datasheets give this as 0.04nm/C for the blue emitters.

Re: Constant Current vs. PWM dimming Revealed

Doug S said:
Interesting data on the blue lux wavelength shift with current. If we take the 380mA as 100% and the 7.6mA as 2%, the shift from 2% to 100% is 5nm. Putting this into perspective, this is the width of a single color bin for the blue emitters. AB21 defines 6 bins for blue emitters, each 5nm wide. There is also a tempco associated with wavelength. The luxeon datasheets give this as 0.04nm/C for the blue emitters.

Yes, compare the shift for 100% and 2% for current dimming.

Next, compare the shift for 100% and 2% for PWM dimming.

Think about what is occuring during this time with heat in the die.

Remember, I held the slug itself within a 2.3C range over the 380mA test range. There is a 15 C/W thermal resistance for the 1W emitter to the slug. 3.6V * 380mA = 1.368 Watt
1.368 Watt * 15 C/W = 20.52 C rise in the emitter die. Add in the slug temperature rise of 2.3C (monitored with a K-type thermalcouple embedded in the slug) and we get to a 22.82 C rise.

22.82 C * 0.04 nm/C = 0.9128 nm shift. We are seeing alot more than this when Current dimming...and there is no peak or dominant wavelength shift when PWM dimming (in the blue).

In fact, the PWM blue dominant wavelength and peak wavelength stay put, and match the emitter running 100% duty cycle (on continously at 1140mA), even clear down to a pulse width of 3.67 millionths of a second.